If you are going to be spending a significant amount of time on a trainer, consider investing in a power-based trainer, or using a power meter on a regular trainer. The absolute measure of work, in the form of wattage, or power, is a useful metric to make your workouts as specific as possible. Keep in mind that although power is an incredibly valuable tool, it’s not the end-all measure of effort—you should also pay attention to the two other elements of training intensity: heart rate (HR) and your rate of perceived exertion (RPE).
Power is your absolute measure of work, heart rate is a variable to measure how your body is responding to that work, and the subjective measurement of effort gives you feedback relative to other sessions.
RPE is often the most important determination of what power you should ride at. For example, if you’re doing an endurance-based session calling for a Zone 3 effort, but your Zone 3 power feels like Zone 4 or Zone 5, and you have an elevated Zone 4 heart rate, it would be a mistake to keep the power in that range. Drop the power, find what feels like Zone 3 for that day. If you don’t have access to power as a metric, refer to the index of heart rate and perceived effort here.